1. Field of the Invention
The present invention relates generally to cardiac stimulation, and more particularly to an implantable endocardial lead which stimulates or senses electrical activity of the heart and which employs a retractable fixation mechanism which can be repeatedly exposed to or shielded from tissue during the process of securing the lead to cardiac tissue.
2. Prior Art
There are generally two types of body implantable leads used with cardiac pacemakers--one which requires surgery to expose the myocardial tissue to which an electrode is affixed and another which can be inserted through a body vessel, such as a vein, into the heart where an electrode contacts the endocardial tissue. In the latter type, the endocardial lead is often secured to the heart through the endothelial lining by a sharpened helix affixed to a distal end of the lead. When the end of the lead contacts the lining of the heart at a desired location, the lead may be secured in place by rotating the lead, thus screwing the helix into the heart tissue.
Such a system has been relatively effective in securing an endocardial lead once the initial location of the lead has been achieved. However, it is known that it is undesirable to expose the sharpened helix while the lead is being inserted through a blood vessel into the heart. Moreover, it is difficult to precisely place an endocardial lead on the first attempt. It is common, therefore, for a physician to repeatedly attempt to place an endocardial lead having a sharpened helix securing means. It is desireable, therefore, to be able to shield the sharpened helix during the insertion of the lead through the vein and between attempts to implant the lead on the heart lining.
In the prior art, various apparatus have been proposed for achieving the desired result. For example, U.S. Pat. No. 3,974,834 to Lawrence M. Kane, discloses an implantable intervascular lead having an accordion-fold sleeve surrounding a helix. The sleeve is retractable to expose the helix and re-expandable to cover the helix in the event the helix is unscrewed and withdrawn. An object of the invention is to permit the lead to be inserted into and guided through a body vessel without snagging the body vessel.
Another attempt at solving these problems is disclosed in U.S. Pat. No. 4,146,036 to Robert G. Dutcher and Albert S. Benjamin. This patent discloses a body implantable, intervascular lead, having a helix fixation means. Apparatus for shielding the helix comprises a moveable piston or shaft located within the coils of the helix. The shaft is spring-loaded in a retracted position by the action of an elastomeric boot which also serves to seal off body fluids from the interior of the lead. A stylet passes through a lumen in the lead and acts against a proximal end of the shaft to force the shaft forward through the helix thus forming a partial barrier and inhibiting the helix from coming in contact with tissue, at least in the axial direction.
In U.S. Pat. No. 4,649,938 to William A. McArthur, an endocardial lead with an extendible/retractable helix fixation means is described. The helix is mounted on a bobbin carried within the electrode tip. The bobbin and helix are retracted into the electrode tip by the action of a spring and are extended out of the tip by pressure from the end of the stylet inserted through a lumen in the lead.